The portions of open sandals or mule-like footwear on which the foot rests, which can be referred to as tread substrates, and also the footwear parts comprising the insoles in closed shoes are conventionally produced from flat materials of the desired thickness by cutting or punching a portion of the material in the shape of the foot. These flat materials can be pieces of leather, plastic or natural material mats or the like which can be laminated or in one piece and, after punching or cutting out, can be specially built up in subsequent production stages. This is frequently done in connection with raised heels, foot cavity shapes (i.e., shapes conforming to the plantar arch region), toe grippers and the like. Despite topographically skillful cutting and optimized packing of the foot shape surfaces, a certain material loss is unavoidable. Thus, in the cost calculations for the materials, the direct costs of the lost material can be decisive, i.e., the waste material can be more or less costly and may or may not be reusable.
When it is desired to produce a sole having a non-uniform thickness, there are production stages involving laminar structures which are assembled to approximate the anatomical variation of the bottom of the foot and, in such production stages, further material and processing costs are involved. It is also possible to produce contoured soles in a single production operation with minimum material losses by hot press or injection molding, but relatively expensive molds are then necessary.
In order to overcome these problems, it has been proposed to manufacture soles by extrusion, an example of which appears in U.S. Pat. No. 3,719,965 in which a wide strip having a variable thickness and having a width roughly corresponding to the foot length is extruded and, subsequently, the sole shape is cut out. Although this process results in a higher production rate of the extruded material, it is still necessary to go through a subsequent production step because the sole shape must be separately cut out.
From this it will be recognized that inconsistent requirements exist in that if a sole is to be produced while adhering to minimum production costs, the sole is cut from a flat material of the desired thickness following the shape of the human foot, giving a uniform sole of the same thickness from the toes to the heel. Alternatively, a sole which attempts to anatomically match the contours of the human foot is desirable, but the production costs rise with the measures taken for obtaining such shapes.